This invention relates to a system for enabling a receiver to decode a transmitted scrambled signal which relates to a programme such that said programme may be obtained in an intelligible manner, wherein in addition to the scrambled signal there are additionally transmitted a subscription key and a pay-per-view key, the receiver functioning in a subscription mode in the event that the receiver is pre-authorised to obtain the programme in an intelligible manner with the scrambled signal being decoded under the control of the subscription key in the presence of an authorising code for the receiver whilst in the absence of such pre-authorisation the receiver is set to function in a pay-per-view mode in which case the programme, on request, is obtained in an intelligible manner with the scrambled signal being decoded under the control of the pay-per-view key. The invention also relates to a receiver for use with such a system.
A number of proposals have previously been made for authorising subscribers or subscribers' receivers to receive scrambled signals in an intelligible manner. Proposals for over-air addressing are contained in European Broadcasting Union (EBU) documents SPB 284, "Television Standards for the Broadcasting Satellite Specification of the C-MAC Packet System", 3rd revised edition of December 1984, SPB 352, "Methods of Conveying C-MAC/Packet Signals in Small and Large Community Antenna and Cable Networks Installation", also of December 1984, and Doc. JIWP 10-11/3-1 of March 1986, which documents are incorporated herein by way of reference. These documents refer to a system, known as System B, which is a shared key over-air addressing system where the scrambling process i.e. the process that renders the picture and/or sound/data unintelligible, is derived from a truly random control word (CW1 or CW2). The control word and any programme data (P) are encrypted using a supplementary key (S) and the resulting cryptogram S(P,CW) is sent over-air in an Entitlement Checking Message (ECM). The Supplementary key(s) together with customer messages or authorisations (M) are further encrypted using a shared distribution key (D) and the resulting cryptogram D(M,S) is sent over-air an Entitlement Management Message (EMM). The shared distribution key is stored within the viewer's conditional access sub-system (CASS) which enables this sub-system to derive the Control Word or words, and to store any authorisations. The EMM has two data streams of which the Unique Customer packets are used to update the CASS in terms of shared distribution key, address, etc. and the Shared Customer packets contain the actual entitlements or authorisations.
The above System B proposal is designed to operate at any one time in one mode, either a subscription mode or a pay-per-view mode. The subscription mode is for general or prepaid services where a subscriber is pre-authorised to receive certain or all programmes in an intelligible manner. The pay-per-view mode allows subscribers to select on impulse those programmes they want to receiver the cost for which is deducted from tokens stored in the CASS. As a result of this if a transmission is operating in the subscription mode and a subscriber is not authorised to receive the programme of that transmission as it may not be in a class (tier) of programmes he has not requested there is no way in which he can receive that programme in an intelligible manner.
A system as described in the opening paragraph is disclosed in the PCT International Patent Application of Publication No. WO 83/04154. The system of this publication overcomes the drawback mentioned above in relation to System B in allowing a subscriber to gain access to a programme in the pay-per-view mode if he is not authorised to receive the programme in the normal subscription mode.